Hi folks, I think I’ve mentioned this PSU before but the question I have might be applicable to other cheap switchers. This lump for the original Atari 520ST is run by an NE5561N control chip with a D45H1 switching transistor. It’s supplied by a multi-tap transformer giving 2x2 feeds of 6VAC and 14VAC with an eventual output of 5V@3A, 12V@30mA, -12V@30mA.
This one will run under load for around 10 minutes then the 5V rail will gradually collapse over the next few minutes. Turn it off and leave for a minute or so and it’s back to normal, repeat as above. Unfortunately there’s no schematic online but I’ve done a mix photo of the control circuits: http://www.binarydinosaurs.co.uk/atariPSU1Hybrid.jpg <http://www.binarydinosaurs.co.uk/atariPSU1Hybrid.jpg> The 4700uF cap in the centre is on the output stage so I changed that yesterday which just makes the eventual collapse longer. Could the actual transformer lump break down over time and recover that quickly? Its 6VAC outputs do go a bit erratic. Also I measured the GAIN input on the NE5561N pin 4 and during the collapse it goes over twice what it is when the 5V rail is stable (7V instead of around 3.4V). I have another running supply to measure against. Any clues appreciated. It’d be nice to feed the 6VAC input from another source but I’m not sure my VARIAC will go that low. — Adrian/Witchy Binary Dinosaurs - Celebrating Computing History from 1972 onwards
